It was proposed to use a polycrystalline sintered compact such as YAG as an optical material for a scintillator. For example, Greskovich et al. (U.S. Pat. No. 5,013,696) disclosed presintering a rare earth oxide mold, which has the composition of 67 mol % Y2O3, 3 mol % Eu2O3, and 30 mol % Gd2O3, in a hydrogen atmosphere at 1625 deg C. for 4 hours followed by HIP (hot isostatic pressing) at 1650 deg C. for 1 hour in an Ar atmosphere of 170 MPa and finally, re-sintering at 1850 deg C. for 2 hours in a wet hydrogen atmosphere (U.S. Pat. No. 5,013,696).
The rare earth oxide sintered compact obtained by presintering has a density ranging from 97% to 99% of a theoretical density and the sintered compact subjected to HIP becomes more compact to reduce the number of pores. The sintered compact subjected to HIP shows not prominent increase of an average crystallite diameter as reported in that the average crystal crystallite diameter after HIP ranges from 2 to 4 μm. Resintering increases the average crystallite diameter of the sintered compact to 25 or more micrometers and squeezes bubbles by using grain growth and clearness of the sintered compact increases. The sintered compact after resintering was reported as having the light loss coefficient of 2.93 cm−1.
Ikesue et al. (J. Am. Ceram. Soc. 79 [7]: 1927-1933. (1996)) reported manufacture of a YAG sintered compact added with Nd. The YAG sintered compact was presintered at 1600 deg C. for 3 hours in a vacuum, subjected to HIP at 1500 to 1700 deg C. for 3 hours under 9.8 or 196 MPa, and finally, resintered at 1750 deg C. for 20 hours in the vacuum. YAG receives SiO2 as an additive for sintering and shows 50 or more micrometers of the average crystallite diameter of the sintered compact by resintering after HIP. They also reported that resintering generates pores, which derived seemingly from Ar of HIP.
In these conventional techniques, resintering at a higher temperature than that of HIP is used for removing pores by increasing the crystallite diameter of the sintered compact. By this way, when a grain boundary moves during grain growth, pores move according to the moved grain boundary to allow pores to move to a surface of the sintered compact. However, as well known, an increase in the crystallite diameter decreases strength of the sintered compact and lowers processibility. In addition, the light loss coefficient of 2.93 cm−1 of the sintered compact of Greskovich et al. is enough for the scintillator, but not enough as the laser material. Ikesue et al. reported that it is difficult to remove pores completely by resintering and, rather, pores are generated during resintering. Pores in the sintered compact cause light scattering and, therefore, the sintered compact containing pores is inappropriate for the laser material.
Next, in the case where a ceramic sintered compact is used as the laser material, there is an advantage in making a transmitting wave front distortion smaller. The transmitting wave front distortion is a quantity expressing unevenness of a wave front when a monochromatic light having an even phase is passed through the laser material. The transmitting wave front distortion may be caused by transformation of a crystal. Therefore, in case of the ceramic sintered compact having a smaller crystallite diameter than that of a single crystal, it is possible to eliminate transformation by escaping it to the grain boundary to make the transmitting wave front distortion smaller. In case of the single crystal, the transmitting wave front distortion is about 0.1 λ.